{-# LANGUAGE BangPatterns #-} {-# LANGUAGE CPP #-} {-# LANGUAGE MagicHash #-} {-# LANGUAGE UnboxedTuples #-} {-# OPTIONS_GHC -Wno-redundant-constraints -Wno-name-shadowing #-} ----------------------------------------------------------------------------- -- | -- Module : GHC.Compact -- Copyright : (c) The University of Glasgow 2001-2009 -- (c) Giovanni Campagna <[email protected]> 2014 -- License : BSD-style (see the file LICENSE) -- -- Maintainer : [email protected] -- Stability : unstable -- Portability : non-portable (GHC Extensions) -- -- This module provides a data structure, called a 'Compact', for -- holding immutable, fully evaluated data in a consecutive block of memory. -- Compact regions are good for two things: -- -- 1. Data in a compact region is not traversed during GC; any -- incoming pointer to a compact region keeps the entire region -- live. Thus, if you put a long-lived data structure in a compact -- region, you may save a lot of cycles during major collections, -- since you will no longer be (uselessly) retraversing this -- data structure. -- -- 2. Because the data is stored contiguously, you can easily -- dump the memory to disk and/or send it over the network. -- For applications that are not bandwidth bound (GHC's heap -- representation can be as much of a x4 expansion over a -- binary serialization), this can lead to substantial speedups. -- -- For example, suppose you have a function @loadBigStruct :: IO BigStruct@, -- which loads a large data structure from the file system. You can "compact" -- the structure with the following code: -- -- @ -- do r <- 'compact' =<< loadBigStruct -- let x = 'getCompact' r :: BigStruct -- -- Do things with x -- @ -- -- Note that 'compact' will not preserve internal sharing; use -- 'compactWithSharing' (which is 10x slower) if you have cycles and/or -- must preserve sharing. The 'Compact' pointer @r@ can be used -- to add more data to a compact region; see 'compactAdd' or -- 'compactAddWithSharing'. -- -- The implementation of compact regions is described by: -- -- * Edward Z. Yang, Giovanni Campagna, Ömer Ağacan, Ahmed El-Hassany, Abhishek -- Kulkarni, Ryan Newton. \"/Efficient communication and Collection with Compact -- Normal Forms/\". In Proceedings of the 20th ACM SIGPLAN International -- Conference on Functional Programming. September 2015. <http://ezyang.com/compact.html> -- -- This library is supported by GHC 8.2 and later. module GHC.Compact ( -- * The Compact type Compact(..), -- * Compacting data compact, compactWithSharing, compactAdd, compactAddWithSharing, -- * Inspecting a Compact getCompact, inCompact, isCompact, compactSize, -- * Other utilities compactResize, -- * Internal operations mkCompact, compactSized, ) where import Control.Concurrent.MVar import GHC.Prim import GHC.Types -- | A 'Compact' contains fully evaluated, pure, immutable data. -- -- 'Compact' serves two purposes: -- -- * Data stored in a 'Compact' has no garbage collection overhead. -- The garbage collector considers the whole 'Compact' to be alive -- if there is a reference to any object within it. -- -- * A 'Compact' can be serialized, stored, and deserialized again. -- The serialized data can only be deserialized by the exact binary -- that created it, but it can be stored indefinitely before -- deserialization. -- -- Compacts are self-contained, so compacting data involves copying -- it; if you have data that lives in two 'Compact's, each will have a -- separate copy of the data. -- -- The cost of compaction is similar to the cost of GC for the same -- data, but it is performed only once. However, because -- "GHC.Compact.compact" does not stop-the-world, retaining internal -- sharing during the compaction process is very costly. The user -- can choose whether to 'compact' or 'compactWithSharing'. -- -- When you have a @'Compact' a@, you can get a pointer to the actual object -- in the region using "GHC.Compact.getCompact". The 'Compact' type -- serves as handle on the region itself; you can use this handle -- to add data to a specific 'Compact' with 'compactAdd' or -- 'compactAddWithSharing' (giving you a new handle which corresponds -- to the same compact region, but points to the newly added object -- in the region). At the moment, due to technical reasons, -- it's not possible to get the @'Compact' a@ if you only have an @a@, -- so make sure you hold on to the handle as necessary. -- -- Data in a compact doesn't ever move, so compacting data is also a -- way to pin arbitrary data structures in memory. -- -- There are some limitations on what can be compacted: -- -- * Functions. Compaction only applies to data. -- -- * Pinned 'ByteArray#' objects cannot be compacted. This is for a -- good reason: the memory is pinned so that it can be referenced by -- address (the address might be stored in a C data structure, for -- example), so we can't make a copy of it to store in the 'Compact'. -- -- * Objects with mutable pointer fields (e.g. 'Data.IORef.IORef', -- 'GHC.Array.MutableArray') also cannot be compacted, because subsequent -- mutation would destroy the property that a compact is self-contained. -- -- If compaction encounters any of the above, a 'CompactionFailed' -- exception will be thrown by the compaction operation. -- data Compact a = Compact Compact# a (MVar ()) -- we can *read* from a Compact without taking a lock, but only -- one thread can be writing to the compact at any given time. -- The MVar here is to enforce mutual exclusion among writers. -- Note: the MVar protects the Compact# only, not the pure value 'a' -- | Make a new 'Compact' object, given a pointer to the true -- underlying region. You must uphold the invariant that @a@ lives -- in the compact region. -- mkCompact :: Compact# -> a -> State# RealWorld -> (# State# RealWorld, Compact a #) mkCompact compact# a s = case unIO (newMVar ()) s of { (# s1, lock #) -> (# s1, Compact compact# a lock #) } where unIO (IO a) = a -- | Transfer @a@ into a new compact region, with a preallocated size, -- possibly preserving sharing or not. If you know how big the data -- structure in question is, you can save time by picking an appropriate -- block size for the compact region. -- compactSized :: Int -> Bool -> a -> IO (Compact a) compactSized (I# size) share a = IO $ \s0 -> case compactNew# (int2Word# size) s0 of { (# s1, compact# #) -> case compactAddPrim compact# a s1 of { (# s2, pk #) -> mkCompact compact# pk s2 }} where compactAddPrim | share = compactAddWithSharing# | otherwise = compactAdd# -- | Retrieve a direct pointer to the value pointed at by a 'Compact' reference. -- If you have used 'compactAdd', there may be multiple 'Compact' references -- into the same compact region. Upholds the property: -- -- > inCompact c (getCompact c) == True -- getCompact :: Compact a -> a getCompact (Compact _ obj _) = obj -- | Compact a value. /O(size of unshared data)/ -- -- If the structure contains any internal sharing, the shared data -- will be duplicated during the compaction process. This will -- not terminate if the structure contains cycles (use 'compactWithSharing' -- instead). -- -- The object in question must not contain any functions or data with mutable -- pointers; if it does, 'compact' will raise an exception. In the future, we -- may add a type class which will help statically check if this is the case or -- not. -- compact :: a -> IO (Compact a) compact = compactSized 31268 False -- | Compact a value, retaining any internal sharing and -- cycles. /O(size of data)/ -- -- This is typically about 10x slower than 'compact', because it works -- by maintaining a hash table mapping uncompacted objects to -- compacted objects. -- -- The object in question must not contain any functions or data with mutable -- pointers; if it does, 'compact' will raise an exception. In the future, we -- may add a type class which will help statically check if this is the case or -- not. -- compactWithSharing :: a -> IO (Compact a) compactWithSharing = compactSized 31268 True -- | Add a value to an existing 'Compact'. This will help you avoid -- copying when the value contains pointers into the compact region, -- but remember that after compaction this value will only be deallocated -- with the entire compact region. -- -- Behaves exactly like 'compact' with respect to sharing and what data -- it accepts. -- compactAdd :: Compact b -> a -> IO (Compact a) compactAdd (Compact compact# _ lock) a = withMVar lock $ \_ -> IO $ \s -> case compactAdd# compact# a s of { (# s1, pk #) -> (# s1, Compact compact# pk lock #) } -- | Add a value to an existing 'Compact', like 'compactAdd', -- but behaving exactly like 'compactWithSharing' with respect to sharing and -- what data it accepts. -- compactAddWithSharing :: Compact b -> a -> IO (Compact a) compactAddWithSharing (Compact compact# _ lock) a = withMVar lock $ \_ -> IO $ \s -> case compactAddWithSharing# compact# a s of { (# s1, pk #) -> (# s1, Compact compact# pk lock #) } -- | Check if the second argument is inside the passed 'Compact'. -- inCompact :: Compact b -> a -> IO Bool inCompact (Compact buffer _ _) !val = IO (\s -> case compactContains# buffer val s of (# s', v #) -> (# s', isTrue# v #) ) -- | Check if the argument is in any 'Compact'. If true, the value in question -- is also fully evaluated, since any value in a compact region must -- be fully evaluated. -- isCompact :: a -> IO Bool isCompact !val = IO (\s -> case compactContainsAny# val s of (# s', v #) -> (# s', isTrue# v #) ) -- | Returns the size in bytes of the compact region. -- compactSize :: Compact a -> IO Word compactSize (Compact buffer _ lock) = withMVar lock $ \_ -> IO $ \s0 -> case compactSize# buffer s0 of (# s1, sz #) -> (# s1, W# sz #) -- | *Experimental.* This function doesn't actually resize a compact -- region; rather, it changes the default block size which we allocate -- when the current block runs out of space, and also appends a block -- to the compact region. -- compactResize :: Compact a -> Word -> IO () compactResize (Compact oldBuffer _ lock) (W# new_size) = withMVar lock $ \_ -> IO $ \s -> case compactResize# oldBuffer new_size s of s' -> (# s', () #)